Use the "swimming" pose in freecam to improve navigation without noClip

[MattSturgeon]

The "swimming" pose is normally used when the player is crawling through a one-block-high space. Or when the player activates the swimming hotkey in water. The swimming pose has the smallest player hitbox available in survival, so when noClip is disabled the swimming pose is required to get through one-block-high gaps and tunnels.

I figured there's probably zero reason to ever need the larger poses while in freecam, so I've made it so that the FreeCamera entity's pose is always set to "Swimming".

We could make the Freecam pose a config option, but I feel like that would needlessly clutter the config menu for little benefit...

Because the eye-hight is matched up with the player's seamlessly, you cannot actually tell your pose has changed until you start moving around and colliding with surfaces.

The new CameraMixin fixes a minor issue I found where setting the pose and moving the camera-entity's y-position confused the camera and caused it to show the camera offset by the eye-height difference and slowly transition back to the correct position. The mixin just sets the camera y-position manually whenever the focused camera-entity toggles to or from a FreeCamera instance.

I originally noticed this issue when you pointed out the no-clip complications over on #25, because FreeCamera using the "standing" pose causes it to collide really easily with the floor the player is standing on.

[hashalite]

Thanks for the PR! Sorry it took me so long to get to these, I recently took a short break from development.